It doesn't matter what unit you use, as long as the units are the same. If you measured in centimeters and it was 60 centimeters by 30 centimeters by 15 centimeters:60 cm * 30 cm * 15 cm = 27000 cubic centimeters

Then you can use the volume conversion page to convert the cubic inches or cubic centimeters into liters.

Now, the weight of the suitcase would be a different matter.

A volume to weight conversion depends on the substance. If you have the suitcase filled with all different things all weighing different weights, it would not be easy to create a conversion factor for that.

But if it is filled with just one thing, such as water, then I can help with that, just need to know what the substance is so I can find out the specific gravity.